Regeneration of spent alkali metal hydroxide solutions from refining of mineral oils



HEWLETT Filed March 15, 1941 REGENERAIION OF SPENT ALKALI METAL HYDROXIDE SOLUTIONS FROM REFINING OF'MINERAL OILS April 13, 1943.

Patented Apr. 13, 1943 REGENERATION OF SPENT ALKALI RIETAL HYDROXIDE SOLUTIONS FROM REFlNllNG F MINERAL OILS Amiot P. Hewlett, Cranford, N. J., assig'nor to Standard Oil Development Company, a corporation of Delaware Application March 15, 1941, Serial No. 383,502

(Cl. 2li-184) 5 Claims.

The present invention is concerned with the rening of mineral oils. 'Ihe invention more particularly relates to an improved process for the regeneration of spent alkali metal hydroxide solutions ,utilized in the renng of petroleum oils. In accordance with the present process, spent alkali metal hydroxide solutions are regenerated utilizing an arrangement and sequence of stages wherein the spent solution is subjected to elevated temperatures and pressures in order to regenerate the same.

In the rening of oils, particularly in the rening of petroleum oils boiling in the motor fuel boiling range, it is desirable to remove the objectionable mercaptan compounds therefrom. This is usually accomplished by treating the oil with a so-called doctor solution comprising a saturated solution of lead oxide dissolved in sodium hydroxide. When the oil is treated with the doc-l tor solution the mercaptan compounds are converted to soluble lead mercaptides. These compounds are then converted to alkyl disuldes by the addition of a reagent which usually comprises free sulfur. When free sulfur is employed the lead is precipitated as lead sulfide. This process for the removal of mercaptans is not entirely satisfactory since -it is relatively dimcult to add the correct amount of sulfur. If an insufcient amount of sulfur be added, soluble lead mercaptides remain dissolved in the oil, which compounds materially affect the burning qualities and the stability of the oil. 0n the other hand, if an excess of sulfur be added, the excess sulfur remains as free sulfur or in the form of organic polysuldes dissolved in the oil and is objectionable since it adversely aifects the corrosiveness, gum stability, color stability and lead susceptibility of the oil. In view of this, various other processes have been suggested for the re- `moval of the objectionable mercaptan compounds from oil. Qne process proposed is to contact theoil with an alkali metal hydroxide solution under conditions to convert the mercaptans to the corresponding alkali metal mercaptides, which constituents are removed with the spent alkali metal hydroxide solution. An operation of this character heretofore has not been entirely successful due to the fact that in order tor remove the mercaptans to the desired extent, it has been necessary to employ a relatively large amount of the alkali metal hydroxide solution.

Furthermore, the large amount of the mercaptide containing alkali metal hydroxide solution necessary was relatively diilicult to regenerate which prohibited its general acceptance as a commercial operation.

I have, however, now discovered a process by which the mercaptide containing alkali metal hydroxide solution secured in the removal of the mercaptans may be readily and eiciently regenerated so that it maybe recycled to the treating dium hydroxide solution, and'that the same is utilized for the removal of mercaptan compounds from a petroleum oil boiling in the motor fuel boiling range. The mercaptan containing petroleum oil is introduced into treating zone I by means of feed line 2. In treating zone I the oil is contacted with the sodium hydroxide solution which is introduced by means of line 3 under,

conditions to remove the mercaptan compounds from the oil. The treated oil free of objectionable mercaptan compounds is withdrawn from treating zone I by means of line Il and handled or further rened in any manner desirable. The spent sodium hydroxide solution is withdrawn from treating zone I by means of line 5 and passed to storage zone 6. From storage zone .6 the spent sodium hydroxide solution is passed to heating zone 'I by means of line 8. In heating zone 'I the temperature of the spent sodium `hydroxide solution is raised and the solution maintained under a pressure at least suicient to maintain the solution in the liquid phase. The solution maintained under an elevated temperature and pressure is I0 by means of line 9. In zone I Il the liquid solution is maintained at the elevated temperature and pressure for a sumcient time period in order to secure a separation between liquid mercaptans and the sodium hydroxide solution. The liquid mercaptans are withdrawn from settling zone Ill by means of line I I, cooled in cooler through pressure release valve I3 and introduced into liquid mercaptan storage I by means oi' line I5. The partially regenerated caustic solution passed to settling zonel l2, passed is withdrawn from settling zone I by means of line I6, passed through pressure release valve I1, and introduced into ilash zone I 8. Temperature and pressure conditions are adjusted to remove overhead by means of line I9 residual quantities of mercaptans and water vapor which are condensed in condenser 2I and introduced into separation zone 20. Liquid mercaptans are withdrawn from separation zone 20 by means of line '22 while the water is withdrawn by means of line 23. The regenerated caustic solution is withdrawn from flash zone I8 by means of line 24, passed through cooling zone 25, and passed to regenerated sodium hydroxide solution storage 26 by means of line 2'1, from where it is recycled to treating zone I by means of line 3. Water or fresh sodium hydroxide solution may1 be introduced into zone 26 by means of line 28. Under certain conditions it may be desirable to introduce the spent heated caustic solution into ilash zone I8 directly by means of line 29. This may be readily secured by adjusting valves 40, 4I, 44, and 43.

The process of the present invention may be widely varied. The respective zones may comprise any suitable number and arrangement of units. Although the present process may be utilized for the regeneration of'spent alkali metal hydroxide solutions utilized in the removal of mercaptan compounds from any oil, it is especially adapted for the regeneration of spent alkali metal hydroxide solutions, particularly sodium hydroxide solutions, utilized in the removal of mercaptan compounds from petroleum oils boiling in the motor fuel boiling range. These oils boil in the general range from about 100 F. to about 450 F. ,f

The spent alkali metal hydroxide solution is heated to a temperature in the general range from about 300 F. to about '700 F. I have found that it is desirable to heat the solution to a temperature of at least 400 F. and that particularly desirable results are secured provided the solution be heated to about 500 F. The pressure at all times should be at least sufficient to maintain the constituents in the liquid phase.

yIt is however preferred to employ pressures from about 25% to about 50% in excess of this figure.

The spent alkali metal hydroxide solution is subjected to the elevated temperature and pressure for a sufficient time period in a separation zone to secure a separation between liquid mercaptans and the sodium hydroxide solution. In general, I have found that this time period should be from about 10 to about 120 minutes, preferably from about 20 to about 40 minutes.

It has been found that the operation of appreciably improved and that the mercaptan removal can be facilitated by allowing some vaporization to take place from the top of the separation zone. If this operation be employed, a mixture of water vapor and vaporized mercaptans is removed by means of line II, while liquid mercaptans are removed by means of line 30 and handled as described. The desired vaporization is controlled by adjusting the respective valves 3| and 42.

In order to. illustrate the invention further, the following examples are given which should not be construed as limiting the same in any manner whatsoever:

Example i A spent sodium hydroxide solution utilized in removal of mercaptan compounds was separated into a number of streams and the respective streams regenerated as described.

In one operation stream A was-preheated to a temperature of about 212 F. and ashed into a flash zone in which the same was treated with stripping steam. In other operations, B to D, inclusive, the respective streams were handled at different temperatures in accordance with the process of the present invention. No venting from the top of the separation zone was employed. The results of these operations were as follows:

Operation From the above, it is apparent that the improvement in the removal of mercaptans which occurred by raising the temperature of the solution to 300 F. was not appreciable, but when the temperature of the solution was raised to 400 F., the improvement in the removal of the mercaptans had increased '76% and was more than tripled by raising the temperature of the solution to 500 F.

Example 2 Tempera- Opcratiun re employed mercaptans (utilizing operation A as a base) Per cent From the above it is apparent that unexpected desirable results are secured in accordance with the present process, particularly when allowing a small amount of vaporization to occur to the Y top of the separation zone.

2. Process as dened by claim l, in which the temperature is in the range from about 400 F. to about '700 F., and in which the pressure is regulated in a manner that a relatively small amount of the mercaptans are vaporized, separatn'g the vaporized mercaptans, and separating a liquid layer of mercapta-ns.

3. Process for the regeneration of a spent alkali metal hydroxide solution utilized in the removal of mercaptans :from petroleum oils, which comprises heating the spent alkali metal hydroxide solution in an initial stage to a temperature in the range from about 300 F. to about 700 F., and maintaining a pressure on the solu-' tion substantially equivalent to the vapor pressure of the solution, whereby a liquid mercaptan phase and an alkali metal hydroxide solution phase form, separating the mercaptan phase, re-

moving the partially regenerated alkali metal hydroxide solution and dashing the same in a secondary stage by substantially reducing thev Stage.

AMIOT P. HEWLEI'I'. 

